Radiographic images such as X-ray images have been largely used in the field of disease diagnosis or the like. To obtain such radiographic images, there have been broadly employed a radiography system in which X-rays transmitted through a subject are irradiated onto a phosphor layer (fluorescent screen), thereby producing visible light and the visible light is irradiated onto a silver halide photographic material (hereinafter, also denoted simply as photographic material) in a manner similar to conventional photography, followed by being processed to obtain visible silver images.
However, a new method of taking an image directly from a phosphor layer has recently advanced instead of an image forming method by use of a silver halide photographic material. In this method, a radiation having been transmitted through a subject is absorbed in a phosphor and then, exciting the phosphor by light or heat energy causes a radiation energy accumulated in the phosphor through absorption to radiate as phosphorescence, and the phosphorescence is detected to achieve imaging.
Specifically, there is known a radiation image conversion method using a photostimulable phosphor (as describe din, for example, Patent documents 1 and 2). This method employs a radiation image conversion panel comprising a photostimulable phosphor layer containing a photostimulable phosphor, in which the photostimulable phosphor of the radiation image conversion panel is exposed to a radiation transmitted through a subject and accumulates radiation energy corresponding to radiation transmission densities of the respective portions of the subject; thereafter, radiation energy accumulated in the photostimulable phosphor is emitted as stimulated emission by exciting the photostimulable phosphor in a chronological order by an electromagnetic wave (exciting light) such as visible light, infrared rays or the like and signals based on intensities of this light are photoelectrically converted to obtain electric signals and the obtained signals are reproduced as a visible image on a recording material such as a photosensitive material or a display device such as CRT.
The foregoing reproduction method of a radiation image has an advantage such that, compared to conventional radiography by a combination of conventional radiographic film and an intensifying screen, a radiation image rich in information content can be obtained at much lower radiation exposure.
The radiation image conversion panel employing such a photostimulable phosphor, after storage of radiation image information, releases a stored energy through scanning by an exciting light, so that storage of radiation image information can be performed, enabling repeated use thereof. Namely, conventional radiography consumes radiographic film for every photographing; on the contrary, the radiation image conversion method repeatedly uses a radiation image conversion panel, which is advantageous in terms of resource protection and economic efficiency.
Recently, diagnostic image analysis requires a radiation image conversion panel with further enhanced sharpness. To achieve enhanced sharpness, for example, there were made attempts of controlling the shape of the formed photostimulable phosphor particles, thereby achieving enhancement of sensitivity and sharpness.
One of such attempt is, for example, a method employing a photostimulable phosphor layer comprised of finely columnar blocks formed by depositing a photostimulable phosphor on a support having a fine concave-convex pattern, as described in JP) 61-142497A.
There were further proposed a method of using a radiation image conversion panel provided with a photostimulable phosphor layer developed by subjecting cracks between columnar blocks formed by deposition of a photostimulable phosphor to a shock treatment (as described, for example, in patent document 3); a method of using a radiation image conversion panel in which cracks were caused from the surface side of a photostimulable phosphor layer formed on the support surface (as described in, for example, patent document 4); and a method in which a photostimulable phosphor layer including voids was formed by vapor deposition on a support, followed by a heating treatment to allow the voids to grow to provide cracks (as described in, for example, patent document 5).
There was also proposed a radiation image conversion panel comprising a photostimulable phosphor layer having formed, on a support, thin columnar crystals with an inclination angle to the normal of the support (as described in, for example, patent document 6).
Recently, there was proposed a radiation image conversion panel using a photostimulable phosphor mainly composed of an alkali halide such as CsBr and activated with Eu, and the activator of Eu enabling to derive enhanced X-ray conversion efficiency which was never achieved before.
When forming a photostimulable phosphor through vapor deposition, means for achieving enhanced luminance include an increase of a phosphor layer thickness. In that case, when continuing deposition, columnar crystal become thicker as the layer thickness increases, resulting in decreased spaces between columnar crystals and leading to disappearance thereof, or causing cracks in the phosphor layer and resulting in insufficient effects of the columnar crystals and leading to insufficient image sharpness.
Patent document 1: U.S. Pat. No. 3,859,527,
Patent document 2: JP 55-012144A,
Patent document 3: JP 61-142500A,
Patent document 4: JP 62-039737A,
Patent document 5: JP 62-110200A,
Patent document 6: JP 02-058000A,
Patent document 7: JP 2003-50298A.